Method of controlling the combustion cycle in a combustion engine

ABSTRACT

In a combustion engine, the amount of working medium is controlled by allowing the engine to suck in an amount of working medium corresponding to full gas operation, of which a portion is then allowed to flow out again without being compressed, while an amount proportional to the current load is retained and compressed, the compression ratio being regulated as a function of the amount of working medium retained. In such an engine there is a movable member (4) for adjusting the compression ratio, the position of said member being controlled by a control means (9). Furthermore, there is at least one spill flow valve (12), the open-time of which is variable as a function of the current load of the engine.

The invention relates to a method of controlling the combustion cycle inan internal combustion piston engine in which a gaseous working mediumis compressed, is combusted and performs work, and in which thecompression ratio is varied during operation.

The invention also relates to the design of a combustion engine.

In a conventional combustion engine, the amount of gaseous workingmedium used in the engine in each cycle will vary with the throttleopening and the engine speed, for example. Since the compression ratiois not changed, this will result in the compression pressure varyingwith the operating conditions for the engine. Consequently, the thermoefficiency of the engine will vary under varying operating conditions.Since the efficiency is generally optimized for high r.p.m. and largethrottle opening, this means that the average efficiency will be low.

It is also known, by German Patent Specification No. 536 930 forexample, to vary the compression during operation. As in conventionalcombustion engines however, all of the working medium drawn in iscompressed. The air/fuel ratio will thus vary under operatingconditions, and therefore the engine will not have an optimum averageefficiency.

The purpose of the invention is to achieve a combustion engine withimproved efficiency under varying operating conditions.

This is achieved according to the invention by regulating the amount ofworking medium by allowing the engine to draw in a maximum amount ofworking medium corresponding to full gas operation, of which a portionis then allowed to flow out again without being compressed and of whichan amount proportional to the current load is retained and compressed,and that the compression ratio is regulated as a function of the amountof working medium retained so that substantially the same compressionpressure is obtained under varying operating conditions.

In an internal combustion piston engine provided with at least onechamber in which there is a first movable member by means of which thegaseous working medium is compressed and after combustion performs work,and in which there is a second movable member for setting thecompression ratio, the position of said second movable member beingcontrolled by a control means, this is achieved by said chamber havingat least one spill flow valve, the open-time of which is variable as afunction of the current load of the engine to make possible, prior tocompression, spilling out of a portion of the maximum amount of workingmedium introduced into the chamber, and that the control means isarranged to set the second movable member in such a position, as afunction of the retained amount of working medium corresponding to thecurrent engine load, that the compression pressure is essentially thesame under varying operating conditions.

In contrast to what is the case in the prior art, the invention adaptsthe working volume of the engine to the current requirement. By virtueof the fact that the engine according to the invention operates underfull gas conditions even at reduced load, improved efficiency is madepossible. This contributes to an improved total efficiency for theengine according to the invention.

The invention will be explained in more detail in the following with theaid of the examples shown in the accompanying drawings.

FIG. 1 shows an engine according to the invention with a reciprocatingpiston,

FIG. 2 shows an engine according to the invention with a rotatingpiston,

FIG. 3 shows a section through a setting means,

FIG. 4 shows a view of the setting means in FIG. 3,

FIG. 5 shows a p-V diagram with various work curves, and

FIG. 6 shows torque curves.

A combustion engine 1, only partially illustrated in FIG. 1, of the typewith reciprocating pistons, has a number of chambers 2 of which only oneis shown in the figure. In each such chamber there is a first movablemember 3, in the form of a reciprocating piston, which is in its upperdead center position in the drawing. In the chamber 2 there is also asecond movable member 4 which is in the form of an axially displaceablepiston in the engine head 5. With the aid of a setting means 6, it ispossible to move the piston 4 to different positions to change thecompression ratio in the chamber 2. In the drawing, the piston 4 is inits lowest position, where the engine is thus operating at a maximumcompression ratio. The piston 4 holds a spark plug 7 which is connectedto the ignition unit 8 of the engine, which provides information on thecurrent engine speed to a control means 9 which is in turn connected toa setting motor 10 in the setting means 6.

The inflow into the chamber 2 is controlled by a valve mechanism 11, andthe outflow is controlled by a valve mechanism 12. The valve mechanism11 is controlled by a valve control means 28, while the valve mechanism12 is controlled by a corresponding valve control means 29. Both ofthese valve control means are in turn controlled by the control means 9.With the aid of the valve control means 28 and 29, which are both of thetype which has already been described in International PatentApplication No. PCT/SE81/00259, corresponding to U.S. application Ser.No. 385,393, filed on May 25, 1982 now U.S. Pat. No. 4,498,352 it ispossible during operation to change the shape of the cams which controlthe valves, so that the opening and closing times for the valves can bevaried as desired, for example as a function of engine r.p.m. and load.This makes it possible to improve the through-flow in the chamber 2. Thedetails regarding the design of these valve control means are given insaid patent application and therefore do not need to be repeated here.Fuel is injected directly into the chamber 2 with the aid of aninjection jet 14.

The amount of working medium is regulated by keeping the outlet valveopen during a controllable portion of the upward movement of the piston3. By allowing introduced working medium to flow out again, it ispossible to compress only as much working medium as is required at thattime. Thus the compression phase will be initiated at differentpositions of the piston 3, depending on the current operatingconditions. The principle for such an engine has been describedpreviously in International Patent Application No. PCT/SE81/00201,corresponding to U.S. application Ser. No. 355,590, filed on May 22,1982. which describes the advantages of such a design in more detail.

Thus in this type of "full gas engine", varying volumes of workingmedium will be compressed, depending on engine load. Without the secondmovable member 4, the engine compression ratio would vary as a functionof load, with the result that the compression pressure would also varyas a function of load. With the aid of the piston 4, it is now possibleto affect the compression ratio of the engine so that a compressionpressure essentially independent of the operating conditions isobtained.

The valves can be given the desired operating curve by controlling thevalve control means 28 and 29 via the control means 9, which can forexample receive information on the size of the load on the engine from agas pedal 30 and by using this information can set the valve controlmeans 28 and 29 for the desired through-flow in the engine. A pressuresensor 15 mounted in the piston 4 senses the pressure in the chamber 2and delivers this information to the control means 9.

Since the movement of the piston 3 is normally synchronized to themovement of the valves in the chamber 2, and since the time for ignitionof the gas mixture is related to a certain position of the piston 3, itis possible, by using the information from the ignition unit 8, todetermine the position of the piston 3 at the moment when thecompression phase begins, i.e. when the valves have just closed. Bysimultaneously registering the pressure in the chamber 2 it is thereforepossible to compute what compression pressure will be able to beachieved in the chamber 2. Such a computation can be carried out in thecontrol means 9, which by comparing the computed value and a commandvalue for the compression pressure can send an impulse to the settingmotor 10 to change the position of the piston 4 so that the desiredcompression pressure in the chamber 2 is achieved. When the engine speedand/or throttle opening is then changed, the pressure in the chamber 2measured at the beginning of the compression phase will also be changed,with the result that the control means 9 sends an impulse for changingthe position of the piston 4. A large throttle opening will mean thatthe piston 4 will be in a high position, while a small throttle openingwill mean that the piston 4 will be in a low position. Since thecombustion engine normally has a specific r.p.m. at which the engine canfunction optimally, the flow through the engine will also be affected bythe r.p.m. With knowledge of how the through-flow varies with the enginespeed it is thereby possible, via the control means 9 and the piston 4,to correct the compression pressure in the chamber 2 for variationscaused by engine speed change. It is of course also possible to providethe control means 9 with other information concerning the currentoperating conditions of the engine in order to control, with the aid ofthe piston 4, the compression pressure in the chamber 2 as a functionthereof.

In applying the invention to a diesel engine, the control means 9 cansuitably receive engine speed information from an injection unit, whichis in turn connected to an injection jet. Otherwise the design can be inprinciple the same as that of the engine in FIG. 1. Contrary to what isthe case in a common diesel engine, according to the invention it ispossible when starting the engine to set the piston 4 in such a positionthat the engine can start from a cold state without the aid of hot bulbsfor example.

Even a Wankel engine can, as can be seen in FIG. 2, be modified so thatthe amount of working medium, instead of being controlled with the aidof a throttle at the inlet, can be controlled with the aid of a specialvalve mechanism 31 which permits, as a function of the operatingconditions, a variable amount of the working medium drawn in to flow outagain. The valve mechanism 31 is also of a type which is described inInternational Patent Application No. PCT/SE81/00259, corresponding toU.S. application Ser. No. 385,393, filed May 25, 1982. and which is soconstructed that the valve 32 will move upwards when it opens. Thus thevalve 32 does not hinder the movement of the piston 3. A suitablepattern of movement for the valve 32 is set with the aid of the valvecontrol means 33 which is in turn controlled by the control means 9.

The valve 32 can open at the earliest when the piston tip 34 has justpassed the inlet 35 and can close at the latest when the piston tip 34has almost reached the valve 32. The piston 4 has in this case, as inthe embodiment according to FIG. 1, the task of compensating for thechange in compression ratio caused by the operating conditions via thevalve 32, so that an essentially constant compression pressure ismaintained. The fuel is injected directly into the chamber 2 with theaid of an injection jet 14.

In engines of the type described in FIGS. 1 and 2, the amount of fuelinjected is so adapted to the amount of working medium to be compressedthat an essentially constant air/fuel ratio is obtained at varyingoperating conditions. The valve times are automatically adapted to theengine speed and load at that time, so that a high degree of filling ismaintained at the same time as the gas exchange work is kept at arelatively low level. The throttle position thus determines how great anamount of working medium is to be retained for compression, the momentwhen the compression phase is to be begun, and the amount of fuelinjected per work cycle. Otherwise a combination of the throttleposition and engine speed determines the selection of the valve times.

To achieve as large a range as possible for the compression ratio, it isadvisable to make the chamber 2 in such a way that it will have a verysmall volume when the first movable member 3 is at its end position. Forthis reason the piston 3 in FIG. 1 has been made in such a way as toleave a very small space when in its end position. As can also be seenin FIG. 1, the transition between the zone of the chamber 2 in which thepiston 3 moves and the zone in which the piston 4 moves, there is a moreor less pronounced constriction, as desired. The piston 4 can also,instead of as is shown in FIG. 2, be placed so that its direction ofmovement forms an angle to the direction of movement of the piston 3.The setting means 6 can also be made as a lever mechanism instead of asa screw mechanism.

The details of the design of the setting means 6 in the embodimentsaccording to FIGS. 1 and 2 are shown in FIGS. 4 and 5. The secondmovable member 4 in the form of an axially displaceable piston runs in alower cylinder 19, which is designed to be mounted in the engine head 5.Via an external thread 20, the piston 4 engages a nut 21 which isrotatably mounted in the lower cylinder 19 and is designed to be kept inplace therein with the aid of an upper sleeve 22 which is in turndesigned to be kept in place in the engine head 5 with the aid of aholder 23 which is to be fixed to the head with the aid of screwfasteners for example. The holder 23 has an axial portion 24 which is inrotationally fixed engagement with the inside of the piston 4. Toachieve the desired rotational fixing, the axial portion 24 can beprovided with axially directed splines 25 for example, which interactwith corresponding cavities in the inside of the piston 4. The nut 21engages a worm gear 27 which is rotationally fixed on a shaft 26. Withthe aid of the setting motor 10, the shaft 26 can be rotated in thedirection desired to move the piston 4 via the worm gear 27 and the nut21 upwards or downwards, as desired. The shaft 26 can to advantage beconnected to several chambers in the engine. By using a step motor asthe setting motor and by using fine pitches, the setting of the secondmovable member 4 can be done with great precision. By also selecting arelatively small diameter of the second movable member 4, the forcesaffecting it as the motor operates can be limited, thereby facilitatingpositional adjustment. The shaft 26 must of course be mounted in asuitable manner in the engine head 5, and the setting motor 10 must ofcourse be mounted so as to be fixed rotationally relative to the enginehead 5.

The embodiments described above can be varied in a number of differentways within the scope of the invention. For example, it is possible toreplace the spark plug 7 with a fuel injection jet in the piston 4 andinstead place the spark plug at another location in the engine head. Thepressure sensor 15 can also be placed differently than as shown.Furthermore, it is normally not necessary to have a pressure sensor inmore than one or two chambers in the engine.

By making the chamber volume variable, it is thus possible to maintainessentially constant compression pressure regardless of the crankshaftangle at which the inlet valve closes, and regardless of the currentpressure in the chamber. The compression pressure and gas temperaturecan be selected to be so high that it will not be necessary to reducethe air/fuel ratio to any significant degree to maintain favorablecombustion conditions, which is essential for maintaining good totalefficiency and cleaner exhaust. Thus the operating conditions of theengine can be improved within a very wide range of engine speeds andloads.

If the engine is turbo-charged, it is not necessary to limit the boostpressure within the range for the optimum gas exchange work for theengine. The compression pressure can instead be limited by increasingthe volume of the chamber. Under such operating conditions the actualcompression ratio of the engine is thus lower than the nominalcompression ratio, despite the fact that the resulting compressionpressure has not been changed. The engine can thus work at a higheraverage pressure without increasing the nominally set maximum combustiontemperature and pressure. Despite the fact that the temperature ratiobetween combustion temperature and exhaust temperature is somewhatpoorer, the total efficiency can still be improved.

FIG. 5 shows in the form of a work diagram (p-V diagram) a comparisonbetween a conventional four-stroke engine and a "full gas engine"according to the invention of the type exemplified in FIGS. 1 and 2. Thesolid line A represents a cycle diagram for an engine at full load andapplies both to a conventional four-stroke engine and to a "full gasengine" according to the invention. The dashed line B represents a cyclediagram for a conventional four-stroke engine at reduced load (approx.30% of maximum load). Finally, the dash-dot line C represents a cyclediagram for a "full gas engine" according to the invention also at about30% of maximum load. The advantages of the engine according to theinvention over conventional engines at reduced load are clearly evidentfrom the figure, making substantially improved efficiency possible bybeing able to keep the compression pressure (the pressure level at thelower end of the left-hand vertical portion of the cycle) at the samelevel as for full gas operation (curve A). The higher compressionpressure also means a higher compression temperature.

It is essential to note in this connection that for a conventionalengine, the best possible work cycle can only be obtained at a certainpredetermined engine speed, while in a "full gas engine" according tothe invention it is possible to achieve the best possible work cyclewithin a large r.p.m. interval. The adjustability of the valvescontributes appreciably to the advantageous result of the invention.

FIG. 6 shows schematically the difference in the shapes of the torquecurves for the different engines. The curve D is for a conventionalfour-stroke engine, while curve E is for a "full gas engine" accordingto the invention. As is clearly evident, the torque in a conventionalengine drops off sharply on both sides of a maximum. In an engineaccording to the invention however, the torque curve is substantiallyflatter, which is largely dependent on the fact that more advantageousvalve times can be used. At high r.p.m., it is possible according to theinvention to avoid the inlet valve closing too early, and thus theengine can be provided with more working medium, which improves thetorque. At low r.p.m., it is possible according to the invention toprevent the outlet and inlet valves being open too long at the sametime, and thus it is possible to achieve better through-flow, which inturn improves the torque.

I claim:
 1. A method of controlling the combustion cycle in an internalcombustion piston engine in which a gaseous working medium iscompressed, is combusted and performs work, and in which the compressionratio is varied during operation, comprising:regulating the amount ofworking medium by operating the engine to draw in a maximum amount ofworking medium corresponding, to full gas operation and then causing aportion of the working medium to flow out again without beingcompressed; retaining and compressing an amount of the working mediumproportional to the current load; and regulating the compression ratioas a function of the amount of working medium retained to obtain asubstantially constant compression pressure under varying operatingconditions.
 2. The method of claim 1 in which retaining and compressingthe amount of the working medium comprises closing a valve controllingthe flow of working medium out of a chamber defined in the engine whenthe amount of working medium in the chamber is proportional to thecurrent load.
 3. The method of claim 2, further comprising determiningthe position of a piston in the chamber and sensing the pressure in thechamber immediately after the valve is closed; regulating thecompression ratio comprising varying the compression ratio in accordancewith the determined piston position and the sensed pressure.
 4. Themethod of claim 3 in which varying the compression ratio comprisesvarying the volume of the chamber.
 5. The method of claim 1 in whichregulating the compression ratio comprises varying the volume of achamber defined in the engine.
 6. The method of claim 1, furthercomprising injecting an amount of fuel into the compressed workingmedium to obtain a substantially constant working medium/fuel ratiounder varying operating conditions.